1. Field of the Invention
The present invention relates to a method for improving the filterability of a microbial broth and in particular of a broth which has been subjected to heating and/or clarification by an enzyme treatment and/or ultrafiltration and/or shear and/or storage. More in particular the broth is a fermentation broth of a polysaccharide producing microorgansim.
2. Description of the Prior Art
Polysaccharide polymers which are produced by fermentation of carbohydrates with suitable polysaccharide producing microorganisms are widely used as water thickeners. Especially in enhanced oil recovery operations they have established a utility as viscosity enhancers of displacement fluids. These displacement fluids are normally aqueous solutions which are pumped into an oil bearing rock formation to displace the oil from the reservoir rock.
Although displacement fluids containing polysaccharides such as those derived from Xanthomonas campestris are now widely used, still a major problem has not been completely solved. This problem concerns the presence of insoluble impurities in industrial grades of these polysaccharide solutions. In the typical commercial production of polysaccharides by e.g. Xanthomonas fermentation, the high viscosity of the fermentation broth precludes complete separation of insoluble material, such as cellular debris and nonviable bacteria from the polysaccharide-containing broth.
As a result, commercial grades of these microbial polysaccharides, i.e. xanthan gums, contain solids which do not dissolve when the xanthan gum is placed in dilute aqueous solution such as that required for polymer flooding in enhanced oil recovery. The presence of these particulate solids in the polysaccharide solution presents considerable difficulty in field application of the polymer flood because they can cause plugging of the rock face and injection water filters. Previous attempts to overcome this plugging problem have included caustic treatment of the polysaccharide solution and subsequent flocculation of the solids, enzyme treatment to bring about chemical decomposition of the solid material in the polysaccharide solution prior to use, and mechanical shear to break up the solids.
The treatment with enzymes of a polysaccharide aqueous solution to obtain a clarified solution has been widely described. From U.S. Pat. No. 4,010,071 a clarification of xanthan gum with a protease enzyme is known. The treatment with protease enzymes has also been described in U.S. Pat. No. 4,119,491. To improve the clarification an enzymatic disintegration is initiated. However before the cell bodies are completely disintegrated the solution is contacted with particles of solid siliceous material at an adsorption-enhancing pH followed by filtering-out the siliceous solids and the partially-disintegrated cell bodies that are adsorbed on them. In U.S. Pat. No. 4,416,990 corresponding to GB Pat. No. 2,085,904, a process is described for enzymatically purifying of a xanthan gum containing as impurities bacterial cell residues and microgels. This process comprises the treating of an aqueous dispersion of the gum with Basidiomycete cellulase.
In U.S. Pat. No. 4,326,037 corresponding to GB Pat. No. 2,065,688, a method is described for enhancing the ability of polysaccharides in aqueous solutions to flow through a porous medium which method comprises contacting the polysaccharides with an endoenzyme which is capable of hydrolyzing at least one of the linkages of the sugar units of the polysaccharides and maintaining the polysaccharides in contact with the enzyme under hydrolysis conditions for a time sufficient to decrease the tendency of the polysaccharides to plug the porous medium yet insufficient to decrease the viscosity of the aqueous polysaccharides by more than 25%.
Efforts are still being made to improve the filterability of a polysaccharide containing aqueous solutions. The term filterability is commonly used to describe the ability of a fluid to flow through a porous medium, and derives from the filtration test employed in the oil industry. From U.S. Pat. No. 4,431,734 corresponding to GB Pat. No. 2,099,008, an enzymatic process is known for the treatment of xanthan gums for improving the filterability of their aqueous solutions. In this process a combination of two enzymes i.e. a polysaccharase and protease is used.